Explanation:
1 km = 1000 m
(hence the term "kilo" which means 1000).
so. he runs 1000 m at the rate of 3 minutes per km, that means 3 minutes per 1000 m.
now, we can see the solution already with our eyes.
but let's do it formally :
the rate or ratio between time and distance must be the same for the same speed.
so,
3 min / 1000 m = x min / 500 m
500 m × 3 min / 1000 m = x min
1/2 × 3 min = x min
3/2 = 1.5 min = x min
so, it takes him 1.5 minutes (90 seconds) to run 500 m.